Ultrasonic testing is a subset of nondestructive testing that involves the propagation of ultrasound waves in an object or material under test. An example ultrasonic system may include an ultrasonic transducer that generates ultrasound waves which are transmitted into an object as the ultrasonic transducer is passed over a surface of the object or near the surface of the object. In a reflection (or pulse-echo) configuration, the ultrasonic transducer may also receive ultrasound waves that are reflected from interfaces and imperfections within the object and then provide the received ultrasound waves to a computing device for analysis. On the other hand, in an attenuation (or through-transmission) configuration, a receiver that is separate from the transducer (e.g., on an opposite side of the object) may receive the ultrasound waves after the ultrasound waves have travelled through the object and then provide the received ultrasound waves to a computing device for analysis.
Within ultrasonic testing, ultrasonic systems may be classified as either liquid-coupled or air-coupled. In liquid-coupled systems, water or another liquid is used as a couplant between the transducer and the surface of the object. Conversely, in air-coupled systems, air or another gas is used as the couplant. Air-coupled systems may thus be considered non-contact systems, since no physical contact between the transducer and the object is required.
In air-coupled systems, a significant amount of the energy of ultrasound waves may be lost due to attenuation in the air. The amount of attenuation in the air is proportional to the frequency of the ultrasound waves. Thus, as the frequency of the ultrasound waves increases, more energy is lost due to attenuation. Furthermore, in some examples, there may be a large difference in acoustic impedances between air and the surface material of an object. This difference in acoustic impedances may also cause a significant loss of energy. For instance, when an ultrasound wave traveling in air impinges on the boundary of solid material, more than 99.9% of the energy of the ultrasound wave may be reflected back due to the mismatch in acoustic impedances. Improvements are therefore desired.